The use of alcohol fuels, particularly methanol, because they are well adapted for the combustion process in internal combustion engines, has been developed long ago in the field of internal combustion engines operating on compressed fuel mixture. More recently, alcohol engines have reached a new stature because of their satisfactory environmental behavior. Alcohol engines emit--with the exception of aldehydes--less pollutants than comparable gasoline engines. The composition of the emitted hydrocarbons has, in addition, a lesser photochemical reactivity with respect to ozone production and thus leads to a lesser ozone damage.
In alcohol engine designs where fuel injection is effected through a suction intake pipe, it has been found that because of the unsatisfactory cold engine start when pure alcohol was used as fuel, only a mixture of 85-90% methanol or ethanol and a corresponding proportion of low-boiling point hydrocarbons made possible a disturbance-free operation. Tests with alcohol engines fueled with methanol have shown, however, that even if a special three-way catalyst system is used, it has to be regarded as extremely critical that--while taking into consideration the required test distance of 50,000 miles--the limit values determined by the California anti-pollution laws (LEV and ULEV) for the formaldehyde emission in the methanol operation are not exceeded. The basic reason for such relatively high raw formaldehyde emissions in methanol driven Otto-cycle engines is the "quench effect" unavoidably appearing at the relatively cool walls of the combustion chamber and the cylinder upon external mixture formation and the incomplete combustion of the fuel/air mixture caused by such "quench effect". These disadvantages are encountered particularly during low-load engine run.
To avoid emission and cold start problems encountered typically in alcohol engines in which fuel is injected into the suction pipe, the so-called stratified-charge engines have been developed in which the fuel is injected directly into the combustion chamber and the combustion is initiated by means of an ignition device constituted by a spark plug or a glow plug. In these arrangements the injection of the alcohol fuel is effected preferably shortly before the piston reaches its upper dead center, so that an intensive contact of the fuel or the already formed mixture with the cooler combustion chamber parts is limited to a relatively small surface of the combustion chamber. This effect is enhanced particularly in case of high-compression engines with high cylinder charging temperatures at the moment of the injection by a rapid vaporization of the injected alcohol fuel. Nevertheless, the mixture layers which depend from the geometry of the injected jet in the combustion chamber, cause significant problems, involved particularly with the use of spark plugs. Spark plugs, however, have a significant advantage over permanently heated glow plugs because the ignition start may be controlled independently from the beginning of the injection. The jet expansion dependent upon the injected fuel quantities leads--apart from the stoichastic oscillations of the jet geometry typical for each injected jet--to significantly varying mixture compositions in the region of the spark plug, involving significantly fluctuating ignition quality, including ignition misses. Since, in addition, a direct impingement of a still liquid fuel on the spark plug has to be avoided, a coordination of the position of the injection jet in the combustion chamber and in relationship to the spark plug has to be so selected that in case of large injected fuel quantities, excessive fuel quantity should not reach the spark plug. On the other hand, in case of lesser injection quantities, for example, during idling, no ignition misses should occur. These are caused by an excessively lean mixture at the spark plug electrodes. The coordination compromises in regard to an acceptable overall operational behavior lead in general to increased emissions of non-combusted or only partially combusted fuel in the partial load range. The full-load behavior is essentially characterized by a significant excess of air required for a complete combustion of the non-homogenous mixture and accordingly, by a small specific output as compared to an Otto-cycle engine. Because of the basically lean operational mode, in such engines, paritcularly at higher engine loads, significantly higher NO.sub.x emissions are to be expected than in Otto-cycle engines with a stoichiometrical mixture and corresponding three-way catalyst control.